Your readings are good, that's where you want to be in the range of phosphates. To answer your question, I don't think you would want your phosphates much lower than that, I have seen many great SPS tanks with slightly higher phosphates in 0.05ppm range. The SPS do need some phosphates or they will pale out and starve. You mentioned you have some new SPS that were brown, I don't think your phosphates are high enough to contribute to the browning. There are a lot of other factors, transferring stress, new environment, water parameters, time, lighting and etc. These all play a role in SPS color. Hope this helps.
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